This invention relates to the field of incinerators. The invention is especially intended for incinerating hazardous gases, such as silane, which are vented from semiconductor manufacturing equipment.
Silane gas is a byproduct of the semiconductor manufacturing process. In the latter process, silane gas is generally made to flow, at or near atmospheric pressure, over silicon wafers for reaction with the silicon. Silane leaving the region of the wafers constitutes a safety hazard because it can ignite spontaneously upon contact with air.
U.S. Pat. No. 4,661,056 discloses an incinerator specifically intended for use in burning silane gas or other gases. This specification hereby incorporates by reference the disclosure of the latter patent. The incinerator described in the latter patent has a first pipe and a second pipe, the pipes being mutually perpendicular, and being welded or otherwise bonded to each other. A central section of the exterior wall of the first pipe is open to the second pipe, so that the two pipes are in fluid communication with each other. The first pipe also has an open end, into which air (or some other gas which supports combustion) flows. Silane gas (or some other gas which is to be combusted) enters the system through the second pipe.
In order to assure uniformity of the semiconductor manufacturing process, the silane gas is conveyed through the semiconductor manufacturing apparatus at low pressure. Thus, the silane gas entering the second pipe is still at low pressure. The air entering through the first pipe strikes a side wall of the second pipe, and establishes a turbulent, centrifugal swirling action which causes the air to mix with the silane, and to break oxide bubbles which may have formed around the silane. The swirling action promotes complete combustion of the silane gas. The air also provides a cooling medium for the gaseous products of combustion.
A spark plug provides additional control over the incineration process. The incinerated gases pass through the remainder of the first pipe, and enter a scrubber. A fan draws the gases out of the first pipe and towards the scrubber.
The apparatus described above has the following potential safety problem. If the scrubber fan fails, the air movement through the incinerator is reduced, and there is a risk that the toxic gases still in the incinerator may escape through its open end. One possible solution to this problem is to provide backup power for the scrubber fan, and to connect the process reactor to the fan, so that the reactor stops if the fan fails.
Clearly, the latter solution does not account for the gases which are still located in the process line. To some extent, the fan will continue to draw gases out of the incinerator even after power is lost, because the fan will continue to operate for a short time due to inertia. The effectiveness of the fan after loss of power depends on various factors such as fan capacity, the size and length of the main duct, the number of lines entering the duct, and the length of the conduit between the process reactor and the incinerator. Reliance on the inertia of the fan is obviously not a satisfactory solution, as there is no assurance that the decelerating fan will operate long enough to convey all remaining toxic gases to the scrubber.
The present invention provides an incinerator with an improved auxiliary evacuation system for removing toxic gases from the incinerator in the event of failure of the scrubber fan.